1. Field of the Invention
The present invention relates to an image processing apparatus and a non-transitory computer readable recording medium.
2. Description of Related Art
In applications for DTP (Desktop Publishing) and such like, when an image having a color is used as a background and an object such as a character and a figure having a color different from the color of background is superposed on the background, image processing called knockout is performed in some cases, the knockout being processing of removing the object part from the background image. When the knockout is performed to an original image in which a black character is arranged on a blue background, for example, an image is generated by combining the character with the background from which the character part is removed.
In the electrophotographic method, monochromatic images are individually formed and thereafter superposed on each other at the same position on an image carrier to be transferred, and thereby, an image having a plurality of colors is formed. Since the object and the background image generated by the knockout have colors different from each other, when the positions to superpose the monochromatic images are shifted, space is possibly generated between the object and the background image. Against this problem, trapping processing is generally performed at the time of rasterizing to slightly overlap the object and the background image.
On the other hand, in a case where image formation is performed by the electrophotographic method, the line width of object such as a character and a ruled line is sometimes larger than the original line width due to a dot gain phenomenon, splattered color materials, crush and such like. In order to suppress such increase in line width, thinning processing has been performed to image data, which was generated by rasterizing, to reduce tone values of the contour of the object (for example, see Japanese Patent Application Laid Open Publication No. 2005-341249).
However, it is difficult to obtain both effects of trapping processing and thinning processing.
For example, the contour of object which was subjected to the trapping processing overlaps the background thereof, and the contour of object has a color different from the color of the object other than the contour. Thus, the contour of object cannot be detected accurately and the accuracy of thinning processing is lowered in some cases. Even if the contour of object can be detected, when tone values of the contour of object are decreased by the thinning processing, the object region is reduced to lose the portion overlapping the background. Thus, space is generated between the object and the background, and the effect of trapping processing is lost.